Surface acoustic wave filter

Abstract

This invention comprises in combination a body of piezoelectric material, means for propagating an acoustic wave in a first mode, means for converting a portion of the acoustic wave energy into an acoustic wave in a second mode, a resonant structure in which the acoustic wave energy in the second mode is resonant, means for reconverting the acoustic wave energy in the second mode into surface acoustic wave energy in the first mode, means for providing a predetermined phase shift of the reconverted acoustic wave energy relative to the original propagated acoustic wave energy, and means for recovering the combined acoustic wave energy of the original and phase shift reconverted acoustic wave energies as an electric output signal.

What is claimed is: 
     
       1. A surface acoustic wave filter comprising: a body of piezoelectric material;   means for propagating a first acoustic wave in a first mode in the surface of said body of said material from an input electric signal;   means for converting a portion of the surface wave energy existing in said first acoustic wave into a second acoustic wave in a second mode;   a resonant structure on the surface of said body wherein acoustic wave energy in said second mode is resonant;   means for reconverting acoustic wave energy in said second mode into surface acoustic wave energy in said first mode;   means for introducing a predetermined phase shift of said reconverted acoustic wave energy relative to said propagated first acoustic wave energy; and   means for recovering combined acoustic wave energy of said first and second phase shift reconverted acoustic wave energies as an electric output signal.   
     
     
       2. The device according to claim 1 wherein said means for propagating said first and second acoustic waves and said means for recovering acoustic wave energy comprises at least one interdigital transducer. 
     
     
       3. The device of claim 1 wherein said means for converting acoustic wave energy in a first mode to an acoustic wave in a second mode, and said means for reconverting acoustic wave energy from an acoustic wave in a second mode comprise surface acoustic wave directional coupling elements. 
     
     
       4. The device of claim 3 wherein each of said directional coupling elements comprises a single strip of metal deposited on a piezoelectric metal surface, said strip extending across a first surface acoustic wave propagation path in which acoustic wave energy is propagated in a first mode, and said strip also extending across a second surface acoustic wave propagation path in which said converted acoustic wave energy in said second mode is propagated within said resonant structure. 
     
     
       5. The device according to claim 1 wherein said resonant structure comprises two interdigital transducers each having tuning means to make each of said transducers a totally reflecting termination to an acoustic wave propagation path, said two transducers being separated from each other by a distance capable of causing resonance at a particular frequency at either end of a path passing through said means for converting a portion of said surface acoustic wave energy into an acoustic wave in a second mode, and said means for reconverting acoustic wave energy in said second mode into surface acoustic wave energy in said first mode. 
     
     
       6. The device of claim 2 further including an output transducer and a pair of first and second separate similar resonant structures including associated converting and reconverting means arranged for coupling acoustic wave energy propagated in one direction to said first resonant structure and for coupling acoustic wave energy propagated in an opposite direction to said second resonant structure and wherein the resonant acoustic wave energy in the first of said resonant structures is coupled to the output transducer from one direction while the resonant acoustic wave energy from said second resonant structure is coupled to the output transducer from said opposite direction. 
     
     
       7. The device according to claim 2 further including an input transducer; first, second, third and fourth output transducers; and a pair of first and second different resonant structures with associated converting and reconverting means whereby acoustic energy propagated from said input transducer is coupled in one direction to said first resonant structure and acoustic wave energy propagated in an opposite direction is coupled to said second resonant structure and wherein resonant acoustic wave energy in said first resonant structure is coupled to a third output transducer associated with said first resonant structure and the resonant acoustic wave energy in said second resonant structure is coupled to a fourth output transducer associated with said second resonant structure and wherein each of said third and fourth output transducers receives acoustic wave energy propagated from said input transducer. 
     
     
       8. The device according to claim 2 further including at least one separate input and output transducer coupled to separate surface acoustic wave ports on one side of a multistrip directional coupler and a first pair of separate similar resonant structures with associated converting and reconverting means, each of said two separate resonant structures coupled separately to separate ports on another side of said multistrip coupler. 
     
     
       9. The device of claim 8 further including at least a second pair of resonant structures with associated converting and reconverting means, each of said second pair of resonant structures being coupled to said same ports as said first pair of resonant structures.